The field of this invention relates to a method and apparatus for calibrating an envelope tracking system, and in particular to a method and apparatus for calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter module of a wireless communication unit.
A primary focus and application of the present invention is the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, the low efficiency is also recognised as being problematic for the base stations.
Additionally, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearising techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion.
Voltages at the output of the linear, e.g. Class AB, amplifier are typically set by the requirements of the final RF power amplifier (PA) device. Generally, the minimum voltage of the PA is significantly larger than that required by the output devices of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output devices, as well as any excess voltage across any pull-down device components due to the minimum supply voltage (Vmin) requirement of the PA.
In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-bit quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
To help overcome such efficiency and linearity issues a number of solutions have been proposed. One technique known as envelope tracking relates to modulating the PA supply voltage to match (track) the envelope of the radio frequency waveform being transmitted by the RF PA. With envelope tracking, the instantaneous PA supply voltage (VPA) of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) of the transmitted RF signal. Thus, since the power dissipation in the PA is proportional to the difference between its supply voltage and output voltage, envelope tracking enables an increase in PA efficiency, reduced heat dissipation, improved linearity and increased maximum output power, whilst allowing the PA to produce the intended RF output.
FIG. 1 illustrates a graphical representation 100 of two alternative PA supply voltage techniques according to the related art; a first technique that provides a fixed supply voltage 105 to a PA, and a second technique whereby the PA supply voltage is modulated to track the RF envelope waveform 115. In the fixed supply case, excess PA supply voltage headroom 110 is used (and thereby potentially wasted), irrespective of the nature of the modulated RF waveform being amplified. However, for example in the PA supply voltage tracking of the RF modulated envelope case 115, excess PA supply voltage headroom can be reduced 120 by modulating the RF PA supply, thereby enabling the PA supply to accurately track the instant RF envelope.
The mapping function between ENV and VPA is critical for optimum performance (efficiency, gain, and adjacent channel power (ACP)). Also critical to system performance is timing alignment between the RF signal and VPA at the PA.
Envelope-tracking can be combined with digital pre-distortion (DPD) on the RF signal to improve ACP robustness. Since the ET system is often a multichip implementation involving function blocks in digital baseband (BB), analogue BB, RF transceiver, power management and PA, consistent ET system performance cannot easily be guaranteed across all devices by hardware. There is therefore a need for some level of transceiver calibration in order to accurately map and centre the ET performance of each device leaving the production line. To make envelope-tracking a cost-effective technology, it is desirable to minimize any extra production calibration time and/or use of external characterisation equipment.
Thus, there is a need for an efficient and cost effective solution to the problem of ET system calibration. In particular, it would therefore be advantageous for an on-board auto-calibration method that compensates for part-to-part variation but preferably does not add any extra testing costs of significance.